Determination of cranio-spinal canal compliance distribution by MRI: Methodology and early application in idiopathic intracranial hypertension

Abstract

Purpose: To develop a method for derivation of the cranial-spinal compliance distribution, assess its reliability, and apply to obese female patients with a diagnosis of idiopathic intracranial hypertension (IIH).

Materials and methods: Phase contrast-based measurements of blood and cerebrospinal fluid (CSF) flows to, from, and between the cranial and spinal canal compartments were used with lumped-parameter modeling to estimate systolic volume and pressure changes from which cranial and spinal compliance indices are obtained. The proposed MRI indices are analogous to pressure volume indices (PVI) currently being measured invasively with infusion-based techniques. The consistency of the proposed method was assessed using MRI data from seven aged healthy subjects. Measurement reproducibility was assessed using five repeated MR scans from one subject. The method was then applied to compare spinal canal compliance contribution in seven IIH patients and six matched healthy controls.

Results: In the healthy subjects, as expected, spinal canal contribution was consistently larger than the cranial contribution (average value of 69%). Measurement variability was 8%. In IIH, the spinal canal contribution is significantly smaller than normal controls (60 versus 78%, P < 0.03).

Conclusion: An MRI-based method for derivation of compliance indices analogous to PVI has been implemented and applied to healthy subjects. The application of the method to obese IIH patients suggests a spinal canal involvement in the pathophysiology of IIH.

FIG. 1

(A) The schematic diagram of compartmental cranio-spinal system. (B) Electrical circuit analogy of the flow dynamics in the cranio-spinal sub-compartments. R1, R2, C1 and C2 are the resistances to flow and compliances of the cranial and spinal sub-compartments, respectively. I2 represents the inertia component of the CSF flow (f_CSF) to and from the spinal canal. The driving force for the flow dynamics is the net blood flow (f_A – f_V) to the cranio-spinal system.

FIG. 2

An example of velocity encoded MRI images of blood flow (image A) and CSF flow (image B) used for derivation of the blood and CSF volumetric flow rate waveforms (images C and D, respectively). Flow in the cranial direction shown in white pixels and in caudal direction is dark. The lumen boundaries of the arteries and CSF space are shown in black and the boundaries of the internal jugular veins lumens are shown in white. The arterial, venous and CSF volumetric flow waveforms over one cardiac cycle are marked by A, V, and CSF, respectively. The net transcranial blood flow (arterial minus venous flow) is marked by A–V (images D).

FIG. 3

Plot of the measured CSF (−) and the modeled CSF (●) flow waveforms derived from one of the subjects. The similarity between two waveforms is 86% (fit%).

FIG. 4

The influence of venous flow dynamics on the derivation of the volume change waveforms. Examples of arterial, venous, and CSF flow waveforms from one of the subjects used for derivation of the volume change waveforms are shown in A. Actual venous outflow is replaced with constant venous outflow in B. Corresponding cranio-spinal, cranial and spinal compartmental volume change waveforms are shown in C and D. A larger peak-to-peak volume change is observed in the cranial compartment when a non-pulsatile venous dynamics is used (D).